C structures and unions

C structures and unions (Reek, Ch. 10) CS 3090: Safety Critical Programming in C

C structures: aggregate, yet scalar • aggregate in that they hold multiple data items at one time • named members hold data items of various types • like the notion of class/field in C or C++ – but without the data hiding features • scalar in that C treats each structure as a unit • as opposed to the “array” approach: a pointer to a collection of members in memory • entire structures (not just pointers to structures) may be passed as function arguments, assigned to variables, etc. • Interestingly, they cannot be compared using == (rationale: too inefficient) CS 3090: Safety Critical Programming in C

Structure declarations • Combined variable and type declaration struct tag {member-list} variable-list; • Any one of the three portions can be omitted struct {int a, b; char *p;} x, y; /* omit tag */ • variables x, y declared with members as described: int members a, b and char pointer p. • x and y have same type, but differ from all others – even if there is another declaration: struct {int a, b; char *p;} z; /* z has different type from x, y */ CS 3090: Safety Critical Programming in C

Structure declarations struct S {int a, b; char *p;}; /* omit variables */ • No variables are declared, but there is now a type struct S that can be referred to later struct S z; /* omit members */ • Given an earlier declaration of struct S, this declares a variable of that type typedef struct {int a, b; char *p;} S; /* omit both tag and variables */ • This creates a simple type name S (more convenient than struct S) CS 3090: Safety Critical Programming in C

Recursively defined structures • Obviously, you can’t have a structure that contains an instance of itself as a member – such a data item would be infinitely large • But within a structure you can refer to structures of the same type, via pointers struct TREENODE { char *label; struct TREENODE *leftchild, *rightchild; } CS 3090: Safety Critical Programming in C

Recursively defined structures • When two structures refer to each other, one must be declared in incomplete (prototype) fashion struct HUMAN; struct PET { char name[NAME_LIMIT]; char species[NAME_LIMIT]; struct HUMAN *owner; } fido = {″Fido″, ″Canis lupus familiaris″}; struct HUMAN { char name[NAME_LIMIT]; struct PET pets[PET_LIMIT]; } sam = {″Sam″, {fido}}; We can’t initialize the owner member at this point, since it hasn’t been declared yet CS 3090: Safety Critical Programming in C

Member access • Direct access operator s.m • subscript and dot operators have same precedence and associate left-to-right, so we don’t need parentheses for sam.pets[0].species • Indirect access s->m: equivalent to (*s).m • Dereference a pointer to a structure, then return a member of that structure • Dot operator has higher precedence than indirection operator , so parentheses are needed in (*s).m (*fido.owner).name or fido.owner->name . evaluated first: access owner member * evaluated next: dereference pointer to HUMAN . and -> have equal precedence and associate left-to-right CS 3090: Safety Critical Programming in C

Memory layout struct COST { int amount; char currency_type[2]; } struct PART { char id[2]; struct COST cost; intnum_avail; } layout of struct PART: Here, the system uses 4-byte alignment of integers, so amount and num_avail must be aligned Four bytes wasted for each structure! currency_type id amount num_avail cost CS 3090: Safety Critical Programming in C

Memory layout A better alternative (from a space perspective): struct COST { int amount; char currency_type; } struct PART { struct COST cost; char id[2]; int num_avail; } currency_type amount id num_avail cost CS 3090: Safety Critical Programming in C

Bit fields Bit field members must be ints • If space is a serious concern, you can select the number of bits used for each member struct CHAR { unsigned ch: 7; unsigned font: 6; unsigned size: 19; }; Layout possibilities (machine-dependent): Note: This won’t work on machines with 16-bit ints ch font size size font ch CS 3090: Safety Critical Programming in C

Bit fields • Portability is an issue: • Do any bit field sizes exceed the machine’s int size? • Is there any pointer manipulation in your code that assumes a particular layout? • Bit fields are “syntactic sugar” for more complex shifting/masking • e.g. to get font value, mask off the ch and size bits, then shift right by 19 • This is what actually happens in the object code – bit fields just make it look simpler at the source level CS 3090: Safety Critical Programming in C

Structures as function arguments • Structures are scalars, so they can be returned and passed as arguments – just like ints, chars struct BIG changestruct(struct BIG s); • Call by value: temporary copy of structure is created • Caution: passing large structures is inefficient – involves a lot of copying • avoid by passing a pointer to the structure instead: void changestruct(struct BIG *s); • What if the struct argument is read-only? • Safe approach: use const void changestruct(struct BIG const *s); CS 3090: Safety Critical Programming in C

Unions • Like structures, but every member occupies the same region of memory! • Structures: members are “and”ed together: “name and species and owner” • Unions: members are “xor”ed together union VALUE { float f; int i; char *s; }; /* either a float xor an int xor a string */ CS 3090: Safety Critical Programming in C

Unions • Up to programmer to determine how to interpret a union (i.e. which member to access) • Often used in conjunction with a “type” variable that indicates how to interpret the union value enum TYPE { INT, FLOAT, STRING }; struct VARIABLE { enum TYPE type; union VALUE value; }; Access type to determine how to interpret value CS 3090: Safety Critical Programming in C

Unions • Storage • size of union is the size of its largest member • avoid unions with widely varying member sizes; for the larger data types, consider using pointers instead • Initialization • Union may only be initialized to a value appropriate for the type of its first member CS 3090: Safety Critical Programming in C

C structures and unions